There are several wrist strap monitor technologies presently on the market. The most used method for ESD-sensitive environments is dual wrist strap monitoring. A dual wrist strap consists of two conductive halves, which in use are electrically connected via a human body. The resistance of a human body indicates proper connection of the operator to the wrist strap. A wrist strap monitor requires a control signal in order to detect conductivity of human body. Existing wrist strap monitors use a control signal of high voltage (up to 12V). A voltage of this level, while safe for an operator, presents danger to the latest very sensitive components. Operators charged to such high voltage who touch sensitive components with their hands or other tool deliver a damaging discharge to the device.
In order to reduce operator's exposure to monitoring voltage, some existing wrist strap monitors utilize pulsed control signals where the control signal is present in the form of pulses. Such a solution leaves the operator without any monitoring for the duration of time between the pulses. This method reduces exposure time, but does not reduce peak voltage exposure present during the pulse.
An alternative method is to supply equal but opposite polarity voltages to the two halves of the wrist strap. The assumption being that the resulting voltage on operator would then be zero. This method fails when there is any difference in the quality of contact to human body by the two halves of the wrist strap or when one half of the wrist strap is damaged. In either case, a significant voltage on the operator may be present.
Another problem with all present schemes of dual wrist strap monitors is that if an operator is connected only to one half of the wrist strap, he or she may be exposed to a higher voltage than is present when a wrist strap is worn properly. This is because the presence of human body resistance is checked by either the voltage drop of the control signal across two halves of the wrist strap or by the leakage of control signal across human body (the wrist). When the circuit is closed, the voltage applied to human body can be marginally tolerable for non-critical applications. However, when the wrist strap is worn with a poor contact between at least one of its halves to human skin, the applied voltage on human body can be as high as 12V. Under this situation the wrist strap monitor that is supposed to assure ESD-free environment creates more exposure to sensitive components than no monitor at all.